Most currently available ski boots, including those enjoying the widest popularity among skiers, comprise a semi-rigid outer shell formed of a synthetic plastic material, enclosing a resilient liner designed to adapt the contours of the wearer's foot firmly to that of the shell, while permitting the wearer a reasonable degree of comfort. Because of practical considerations, the outer shells are made in a limited number of different sizes, each size of shell being intended to cover several different foot sizes. In order to custom fit the feet of a wearer to a shell of appropriate size, there are employed liner assemblies of different types, all of which are intended to secure firm engagement of the wearer's foot to the shell of the boot while at the same time avoiding pressure points which might cause discomfort. A wide variety of liners has been used, including assemblies comprising bags containing a flowable, putty-like material, or resilient plastic, fibrous or other non-flowable material, all of which are usually installed at the factory and thus lack the capability for a true custom fit to the wearer's foot. Other liner systems include the use of an injected foamable material installed in a dealer's shop and intended to fill the void between the wearer's foot and the outer shell. Such systems have several disadvantages, including the necessity for very close control of the amount of foamable material injected, and practical difficulties arising from the heat, odor, and clean-up problems encountered during the foaming procedures.
Another liner system, as described in our U.S. Pat. No. 3,798,799, issued Mar. 26, 1974, uses a liner assembly including a resilient inner padding member having an outer surface which defines one or more cavities between the padding member and the inner surface of the shell. Into these cavities, there is injected a non-compressible thermo-plastic fitting material which is flowable during injection but thereafter hardens sufficiently to give firm support to the foot while affording comfort for skiing.
In one embodiment of the system disclosed in U.S. Pat. No. 3,798,799, intended for use with a rear opening boot, the liner assembly does not itself cover the entire surface of the wearer's foot and ankle enclosed within the shell, the liner being open at the back for removal of the wearer's foot through the rear opening of the boot. The rear opening, which is closed by means of a vertical tongue member pivotally connected at its bottom to the shell, is provided with a pad, which, when the tongue is in the closed position, abuts the rear portion of the wearer's ankle and serves a padding function for this portion of the wearer's foot. The liner attached to the tongue member is an integral portion thereof, being installed at the factory, and is, therefore, designed to accommodate a foot of average or typical contour. The custom fitting aspect of the liner assembly is provided by a bladder which covers the wearer's instep area and the sides of his ankle, into which bladder there is injected a thermo-plastic material under sufficient pressure to fill the space between the wearer's foot and the inside surface of the shell. Although the custom fit produced by the described liner system is generally satisfactory, there is occasionally encountered some difficulty in producing the desired close fit in the rear portion of the ankle above the heel and in the vicinity of the Achille's tendon area, particularly in the case of skiers having ankle or calf contours which vary markedly from the average.